This invention relates to hot air balloons and more particularly to use of solar energy for heating the hot air within the balloon.
Hot air ballooning has become an increasingly popular sport in recent years. In virtually all balloons, the hot air required for buoyancy is generated by a burner which uses propane as a fuel. Typical flight durations with these balloons are from one to two hours. However, by carrying a very heavy overload of fuel, a few flights over twelve hours have been possible. These long flights are for special record-breaking attempts and are not practical for the sport balloonist.
While relatively short flights of one or two hours may be acceptable to many balloonists, it would be most desirable to make longer flights as is commonly the case with gas balloons such as those containing hydrogen or helium. With gas balloons, typical flight durations are more in the range of five to fifteen hours. To make hot air balloons fly, it is necessary to heat air within the balloon sufficiently so that the mass of internal air is less than the mass of the surrounding air by an amount equal to the mass of the balloon and its load. In addition to heating the air with propane or other sources of fuel, it has been known to use solar energy to heat balloons. It is a natural consequence of flying during sunlight hours that some energy will be absorbed by the balloon skin, particularly if it is dark colored, making some contribution toward heat build up. In most cases this is trivial or insignificant.
Two balloons are known to have been designed and built with he intention of capturing sufficient solar energy to fly under complete solar power. One was of very large size (Barnes) and built of a black, heatabsorbing fabric. The efficiency of solar heat collection was relatively low, but because of the great size of the balloon, it was able to make sustained flights using solar heat only. The Barnes balloon was much too large and cumbersome to be practical for sport flying.
In a design by Michaelis, an outer envelope of transparent material and an inner balloon of black material having 80% of the volume of the outer balloon was constructed. While this balloon flew successfully and had only about half the volume of the previously described design, the balloon was quite fragile and was substantially more difficult to fly, again rendering it unsuitable for general sport flying.